This invention relates to apparatus for measuring the state of charge of lead acid batteries, such as may be employed in an electric vehicle, and for providing output data indicative of battery charge.
Decreasing supplies of, and increasing prices for, refined petroleum products such as diesel fuel and gasoline have prompted increased interest in development of an electric vehicle suitable for short distance (i.e. less than 125 miles) travel. Because a sizable number of conventional internal combustion engine automobiles and trucks are only used to traverse such short distances, use of electric vehicles in their place could help lessen domestic dependence on expensive imported oil, as the energy required for electric vehicle battery charging could be supplied by hydroelectric or nuclear power stations.
The maximum range of the electric vehicle is dependent on the total charge capacity of vehicle batteries and battery charge depletion per mile in exactly the same manner that the range of a conventional internal combustion engine vehicle is dependent on fuel tank capacity and fuel consumption per mile. However, unlike conventional petroleum-fueled vehicles whose fuel tanks can quickly be refilled in a matter of minutes, recharging of electric vehicle batteries usually requires several hours. Therefore, an accurate indication of battery charge capacity is required to apprise electric vehicle user personnel of remaining battery charge so that the electric vehicle is not driven a distance beyond that which would permit safe return to a home base, or such other location where battery charging can readily be accomplished. To simplify electric vehicle operation, it would be desirable to display remaining battery charge in an analog fashion much the same way that the quantity of remaining fuel is displayed by conventional internal combustion engine vehicle fuel gauge.
Traditional means for determining the state of charge of secondary (i.e. rechargeable) batteries, such as the type used in electric vehicles, have included current integrating devices such as the electrochemical coulometer. The electrochemical coulometer determines the total charge, that is ##EQU1## passing through a shunt circuit, by depositing an amount of indicating material, such as silver, at one side of an electrolysis cell proportional to the amount of charge passed during a given interval. Resetting of the electrochemical coulometer occurs during battery charging as battery charge current carries indicator material to the opposite side of the cell.
Electrochemical coulometers suffer from the disadvantage that the indication of battery charge capacity they provide does not vary in accordance with battery age or temperature. A "full charge" indication by the electrochemical coulometer may be particularly inaccurate at low battery temperatures, as battery charge capacity decreases substantially as battery temperature decreases. Battery charge capacity also decreases as battery age increases.
To remedy the disadvantage of such traditional means for determining secondary battery charge capacity, the present invention provides an indication of battery charge capacity in accordance with battery resistance, a dynamically varying parameter which is dependent on battery temperature and age.